The logarithmic-length conjecture for binary words with a prescribed number of subsequences

A binary word is a finite word over the alphabet {A,B}\{\mathsf{A},\mathsf{B}\}. For a binary word w\mathfrak{w}, let P(w)\mathsf{P}(\mathfrak{w}) denote its number of nonempty subsequences.

Logarithmic-length conjecture. For every integer n1n\geqslant 1, there exists a binary word w\mathfrak{w} such that

w=O(logn)|\mathfrak{w}|=O(\log n)

and

P(w)=n.\mathsf{P}(\mathfrak{w})=n.

The Fibonacci-type alternating words show that any binary word with exactly nn subsequences must have length Ω(logn)\Omega(\log n), so the conjecture asks whether this lower bound is sharp up to a constant factor. The source does not provide a resolution.

Sources & referencesView supporting material

Primary source

Radosław Żak, “Finding binary words with a given number of subsequences”, arXiv:2210.00342 (2022).

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